Datasheet L6565 (STMicroelectronics) - 7

L6565 APPLICATION INFORMATION
Quasi-resonant operation in offline flyback converters lies in synchronizing MOSFET's turn-on to the transform- er's demagnetization. Detecting the resulting negative-going edge of the voltage across any winding of the transformer can do this. The L6565 is provided with a dedicated pin that allows doing the job with a very simple interface, just one resistor. Variable frequency operation - as a result of different operating conditions in terms of input voltage and output current - is inherent in such functionality. The system always works close to the boundary between DCM (Dis- continuous Conduction Mode) and CCM (Continuous Conduction Mode) operation of the transformer. The op- eration is then identical to that of the so-called self-oscillating or Ringing Choke Converter (RCC).
Detailed Device Description
Internal Supply Block (see fig. 12) A linear voltage regulator supplied by Vcc (pin 8) generates an internal 7V rail used for supplying the entire IC, except for the gate driver that is supplied directly from Vcc. In addition, a bandgap circuit generates a precise internal reference (2.5V±1% @ 25°C) used by the control loop to ensure a good regulation with primary feed- back technique. In figure 12 it is also shown the undervoltage lockout (UVLO) comparator with hysteresis used to enable the chip as long as the Vcc voltage is high enough to ensure a reliable operation.
Figure 12. L6565 internal supply block
+Vin Vcc 8 - + UVLO LIN. REF. REG. 2.5V 7V bus 7/17